The diverse array of benefits space has provided to our activities on Earth has greatly increased over the last few decades. Over this time period, many space applications have been turned into profitable commercial enterprises and important civil programs, bringing with them immense change and incredible socioeconomic benefits. Space-based remote sensing has revolutionized human and environmental security, while space-based navigational systems have created massive efficiencies in the global transportation network and together with satellite communications provided the foundation for globalization.

More recently, we have also learned that the space environment is fragile, and that our actions and activities can have long term consequences. Sustainability of the space environment is the responsibility of all actors in space, and only by working together can its benefits be assured for future generations. The foundation to this sustainability is international civil cooperation in and sharing of space situational awareness data.

Limited Natural Resource
An increasing number of States and private operators are realizing the benefits space has to offer and the number of satellites providing space-based services is accelerating. Sixty-four States or private entities now own or operate satellites in Earth orbit, including such recent notables as Algeria, Nigeria, Venezuela, and Vietnam1.

Ten States have demonstrated independent capability to place satellites into Earth orbit2. Since the launch of Sputnik in 1957, the satellite catalog of objects in Earth orbit has grown to almost 13,000 objects, with another almost 5,000 objects tracked but not cataloged3. Over 90 percent of the catalog consists of debris, dead payloads, and spent rocket stages used to put satellites in orbit, with the remaining few percent being operational satellites.

There is an inherent danger in this recent explosion in the use of space. Like any other limited natural resource, mismanagement and overexploitation can degrade or even destroy the long term sustainability of the space environment. (In example, see the sidebar extracted from that daily SatNews regarding a recent satellite collision.)

The first counter argument that is normally heard in response to this is the big space theory: space is so vast that it is hubris to think that humanity can have any serious impact. Indeed, the volume of Earth orbit from the upper edges of the atmosphere out to geostationary orbit is roughly 5,000 times the volume of the Earths oceans and atmosphere combined. But this argument ignores some fundamental facts that differentiate the space domain from the domains of land, sea and air on Earth.

The driving force behind these facts is orbital mechanics. The physics of gravitational attraction and Keplerian motion dictate how satellites move as well as which orbits are especially beneficial. In particular, geostationary orbit and Sun-synchronous orbits have proven to be the two regimes where the most satellites, and thus satellite services, are concentrated.

Geostationary orbit is a thin racetrack running around the Equator 36,000 kilometers above the surface of the Earth. It is here that any application which requires a generally fixed position relative to the Earths surface and large field of view must reside. Satellite television, big-picture weather, and communications relay are the most prominent civil and commercial applications.

Sun-synchronous orbit is a set of inclinations between 96.5 and 102.5 degrees which, when combined with a specific altitude, allow the orbits for satellites in these inclinations to precess around the Equator opposite the Earths rotation. This means that these satellites overfly the same point on the Earth at repeatable intervals with the same solar lighting conditions each time which makes Sun-synchronous orbits especially useful for applications such as optical remote sensing, mapping and environmental measurement.

All satellites must remain in motion; otherwise they will succumb to the pull of Earths gravity and fall back towards its center. The speed a satellite must maintain is a function of its altitude: closer to the Earths atmosphere the pull of gravity is stronger and satellites here must move faster to maintain their orbit than those higher up. This pull of gravity combines with the Earths atmosphere to dictate another important parameter  orbital lifespan.

The length of time an object in Earth orbit remains there is a function of its altitude above the Earth, and it is not a linear scale. An object at 300 kilometers of altitude has a lifespan measured in a few months, while the lifespan of a one at 600 kilometers is measured in several years.

Above 1,000 kilometers the lifespan of objects is measured in millennia. This permanence means that actions taken in orbit can have consequences for a very long time. Automobile accidents in space cannot be simply cleared away to make the highway safe again.

The most talked about event in the realm of space security in recent years was the Chinese anti-satellite test in January 2007 which destroyed the Fengyun-1C weather satellite5. A significant fraction of the 2,800 pieces of trackable debris created by this event was thrown to altitudes greater than 1,000 kilometers. Two years later, less than 50 pieces have re-entered the Earths atmosphere. Multiple satellites have been forced to conduct collision avoidance maneuver due to close approaches with pieces of Fengyun-1C debris, including NASAs $400 million Terra satellite in Sun-synchronous orbit6.

As significant an event as this was, it is nowhere near the worst event imaginable. Outside of a massive nuclear detonation in space, the worst event would be the energetic fragmentation of a satellite in the geostationary belt, either from a massive internal explosion or from a collision with another object. A very thorough analysis of exactly this scenario was the subject of a recent AIAA paper8. The authors simulation of a what could have been collision from the close approach of Cosmos 1961 and Eutelsat W6 in August 2008 shows that the nearly 5,000 pieces of trackable debris would have spread throughout the entire geosynchronous belt within 36 hours. Two days after the event, the wide variation in differential velocity imparted in these pieces would have spread them through many orbital regimes, with the potential to impact all orbits over the coming decades.

While this was just a fictional event, the chances of it happening in reality are increasing every day. Eutelsat W6 mentioned in the simulation above was under full control and thus able to maneuver to avoid the collision, if necessary. Of the 1,150 known objects in the geostationary belt, only 243 of those are under full control (both longitude and inclination), with only an additional 365 satellites under partial stationkeeping control10. The number of objects drifting or captured by libration points, and thus unable to maneuver to avoid collisions, comprise more than half of the population.

In Sun-synchronous orbit, the situation is even worse. Of the more than 4,000 tracked objects in this region, less than 150 are operational satellites. More important, due to the nature of Sun-synchronous orbits, the vast majority of the objects in these orbits cross paths at the Poles every 45 minutes, with head-on closing velocities approaching 14 kilometers per second.

Three-Part Solution
Given the above analysis, it is in the interests of all space actors to invest in the long term sustainability of Earth orbit and in particular those orbits that provide essential benefits. The general set of possible solutions to this problem can be broken down into three areas: debris mitigation, space traffic management, and debris removal. Each has its own advantages and disadvantages that works in concert with the others.

Debris mitigation is currently the area with the most progress and focus within the international community. Its goal is to limit the amount of debris generated in the launch, on-orbit and re-entry phases of space operations. Several of the major space agencies around the world formed the Interagency Space Debris Coordination Committee (IADC) in 1993. As the result of more than 15 years of work, the IADC generated a set of debris mitigation guidelines in 2004. These guidelines were eventually endorsed by the United Nations and several States are currently in the process of implementing them with national legislation and economic mechanisms.

While debris mitigation is an important step, it does not address the problem of the existing debris on orbit. Recent studies have indicated that even without additional satellites placed into orbit, the existing population of orbital debris is likely to increase through collisions between each other12. The only way to tackle this problem is by developing methods of actively removing debris from orbit. While the technical and economic feasibility of this is currently the subject of an on-going IAA study due to report in 2009, the scope of such a solution need not be extensive. Studies have also showed that removal of even five of the most dangerous objects each year was enough to stabilize the existing on-orbit population13.

In the meantime, space actors must turn to methods of minimizing the effects of existing debris on their spacecraft and services. This is the primary goal of space traffic management (STM). Like air traffic management, the goal is to prevent collisions between active satellites and pieces of debris or other satellites. Two techniques form the backbone of STM: conjunction assessment, the prediction of close approaches and associated probability, and collision avoidance, maneuvers undertaken to prevent high probability collisions.

Currently, the only international entity performing a substantial level of STM is the United States military. It uses the extensive satellite catalog derived from its global network of optical and radar sensors to screen a limited list of important military and civil satellites for conjunctions. However, sensor and analytical capacity limitations prevent the expansion of this service to include all operational satellites under the control of the United States, let alone the world. And while the U.S. military is pursuing technological upgrades to add capacity, national security limitations will probably prevent it from performing this service for the world in the foreseeable future.
Many space actors are beginning to realize the eventual need for a formal international space traffic management system even though the technical and political mechanisms to enable this are far from complete. The most significant need is the development of an international civil space situational awareness (SSA) system. Space situational awareness evolved from the military concept of space surveillance. While space surveillance concentrates on tracking mainly the position of objects in space, military SSA seeks to add additional elements to develop a persistent, predictive picture of the space environment that includes adversarial intent.

But space situational awareness is not the sole domain of the military. Just as many other militarily useful types of data also have important civil applications, so does SSA. Currently, satellite operators have an excellent idea of where their particular satellite(s) are but little to no picture of whats going on around them. They are, in essence, driving a car with the windows blacked out while looking at a GPS unit, oblivious to other traffic around them and relying on the big space theory to ensure that no collisions happen.

SSA as the Foundation
International civil space situational awareness is a way of correcting this situation. Its goal is to provide a base level of information about the position of all relevant objects in space to all actors to enable intelligent and efficient use of space. An important distinction is the difference in requirements between military and civil SSA. At its most basic level, civil SSA only needs to provide the position of an object in space and the entity to contact in regards to that object. It does not need to provide the capability to fully characterize the capabilities of that object nor determine its intent. In this way international civil SSA can provide a needed service while simultaneously addressing the security and privacy concerns of both governmental and commercial operators.

The international aspect of such a system lies not only within the distribution of data but also in its collection. By its nature, the task of providing SSA requires a global solution. Observing a satellite at only one point in its orbit only gives you an accurate location at that point; in order to be able to accurately predict where it will be in the future you need multiple observations scattered around the entire orbit. Low Earth orbiting satellites are constantly in motion, making multiple orbits of the Earth each day while the Earth rotates underneath them, making it folly to try and produce an accurate orbit from a single location on the Earth.

Geostationary satellites pose a different problem. Sitting over a certain spot on the Equator, a tracking station within its field of view can track it throughout its entire orbit in one day. But the same object can only be tracked from territory located within its field of view; locations on the other side of the planet will never be able to track it.

To date, the few militaries around the world that have tried to tackle this problem have done so with a geographically distributed network of optical telescopes and radar facilities supplemented with mobile tracking ships. However, maintaining and coordinating these far-flung installations is an extremely expensive undertaking. Only the United States military has successfully developed the capability to maintain an accurate catalog of both low Earth and deep space orbits. Even that capability has severe restrictions, stemming from its foundations on the polar-orientated missile warning network, paucity of deep space telescopes, and almost complete lack of Southern-hemisphere coverage.

A true SSA system also needs to be more than just tracking installations on the Earth. Space weather information is another crucial piece of data. The fluctuations in Solar activity not only determines the expansion and contraction of the Earths atmosphere, which dictates the decay rate of low Earth satellites, but also can generate massive solar storms and particle emissions. These severe space weather events can degrade or destroy satellites and even affect terrestrial power grids and communications networks.

Current SSA and STM Efforts
There are two solutions currently under development that attempt to address the need for both SSA and STM. The United States Air Force was authorized by the U.S. Congress in 2003 to provide satellite tracking data to entities other than the US government where it did not adversely affect national security15. This program, known as Commercial and Foreign Entities (CFE), started with the creation of the Space Track website and transition from the previous website operated by NASAs Orbital Information Group (OIG)16. This website provides unclassified positional data, called Two Line Elements (TLEs), on much of the satellite catalog maintained by the U.S. military to anyone who creates a login. Currently, the U.S. military is working on Phase 3 of the CFE Program, which is intended to provide advanced services to customers. These services could possibly include conjunction assessment, collision avoidance, and anomaly resolution and may or may not include a service fee.

Originally, CFE was planned to be almost entirely a one-way street, with outside entities providing data to the U.S. military who would then do all calculations and analysis internally. Results and recommendations may or may not be transmitted to outside entities. A service implemented in this manner would not meet the international needs for neither SSA nor STM as it would not allow outside entities to make their own calculations and risk analysis. Additionally, national security considerations would most likely limit the service to commercial entities and friends and allies. Recently the National Security Space Office (NSSO) took charge of the program and there have been indications that CFE Phase 3 may be implemented in a much more open manner, including some two-way data transfers and open participation.

A second promising service is the Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space for Geosynchronous (SOCRATES-GEO) service offered by the Center for Space Standards and Innovation (CSSI)17. Based in Colorado Springs, CSSI is a research arm of Analytical Graphics, Inc. (AGI), makers of Satellite Tool Kit (STK). SOCRATES-GEO is a partnership between CSSI and several commercial GEO providers where voluntary owner-operator positional data and maneuver schedules are provided to CSSI by the commercial partners. The CSSI analysts and software mix this information with data pulled from the U.S. militarys public satellite catalog on debris and other objects. The resulting web service gives the commercial owner-operators daily predictions of all conjunctions and access to additional resources to help make collision avoidance decisions.

Recently, another important relationship was developed between CSSI and the International Scientific Optical Network (ISON). ISON is a network of 25 optical telescopes located at 18 scientific institutions across the globe19. Managed from the Keldysh Institute of Applied Mathematics in Moscow, ISON has the capability to track satellites in all orbital regimes and provide very accurate data. This capability was highlighted in several cases recently with the most recent example involving the now-defunct INSAT-1B. At the beginning of February 2009, INSAT-1B drifted through the SES ASTRA 1 cluster at 19.2° E longitude. SOCRATES-GEO originally warned SES ASTRA that it was predicted to pass within 108 meters of ASTRA 1F, based on public TLE data from the U.S. Air Force. However, CSSI was able to use ISON data to refine the close approach to just inside 3 kilometers. This allowed SES ASTRA to plan the appropriate avoidance maneuver, which increased the miss distance to just over 14 kilometers.

On-going talks between ISON, CSSI and the commercial providers are underway to determine if and how to more fully integrate the ISON data into the SOCRATES-GEO system. The added benefit would be greatly improved accuracy on the debris and other objects without owner-operator data. While the US military does not list the accuracy for the TLEs in its public database, independent analysis puts the error for geosynchronous objects somewhere between 50 and 75 kilometers20. ISON is able to provide data in some cases down to just a few kilometers of error, making the resulting conjunction analysis vastly more accurate and useful.

A Glimpse of the Future
As they currently stand, both CFE and SOCRATES-GEO are laudable efforts with unique advantages but still fall short of what the international community needs to maintain the sustainability of Earth orbit. As long as it is backed by the U.S. government, CFE will be seen as untrustworthy by some and unable to participate with others or cooperate in a bi-directional manner. SOCRATES-GEO only provides the data and services to those commercial providers who are partners, and is limited to just geosynchronous orbit.

Looking forward, we can outline what the desired solution could be. It would necessarily be an international solution, where any governmental or private entity with a demonstrated need could access the data. This would be contingent on these same entities providing data to the system, either owner-operator data on their satellites or data collected from one or more sensors that they operate. Each entity would be free to provide only the data that met their individual security and privacy concerns. The need for geographically distributed sensors would create incentive for those States in key locations to provide sensor data while simultaneously eliminating the need for any one State to spend incredible amounts of money on their own sensor network.

Each participating entity would be able to access all the pooled data and make their own independent analysis. Those entities without indigenous capability to analyze the data would have access to conjunction assessment, collision avoidance, and anomaly resolution services. The entire system could be managed by an international non-profit, possibly modeled after the Internet Corporation for Assigned Names and Numbers (ICANN), which has a mix of governments, non-governmental organizations, and private businesses on its Board of Directors. This would ensure that the interests of all entities are represented equally and that no one government had exclusive control over the system.

In addition to addressing the immediate need for data in support of conjunction assessment and collision avoidance, such a system would also have many other benefits. It would increase the available data on space debris and the space environment, enabling additional research into the problem and potential solutions as well as educating all space actors on the severity of the problem. Such a system would also enhance the transparency and cooperation among States which could provide stability and reduce the likelihood of conflict resulting from fear, paranoia, or mistakes. And it could also serve as verification for a potential Code of Conduct in space, setting the stage for future space governance models.

Such an international civil space situational awareness system is not a dream. All of the essential technical elements exist and there is a demonstrated need. What is lacking is the political will on behalf of both private industry and States to come together and create what is truly needed for the benefit of all humankind.